The observation of an eye, particularly of a fundus of the eye, usually serves for making a diagnosis of diseases of the eye as well as for the treatment of such diseases. In particular, age-related macular degeneration or diabetic retinopathy belong to these diseases. An early recognition of these diseases by diagnostic methods such as direct observation of the retina by a physician, fundus images, fluorescence images or optical coherence tomographies need special optical technologies since the fundus of the eye must be observed and imaged through a small opening of an eye pupil, respectively.
For the treatment of diseases of the retina frequently a laser irradiation is carried out on symptomatic regions of the retina. For this treatment a laser radiation must be coupled into the eye through the eye pupil and focused on the location to be radiated. During such a treatment the observation of the retina is necessary, as well.
Optical elements used directly at the eye for this purpose play a decisive role for the accuracy of an optical image of the fundus of the eye. For the observation of the fundus of the eye a slit lamp is used by standard. Using an ophthalmoscoping lens as explained for example in U.S. Pat. No. 5,526,189 the fundus of the eye is imaged in an intermediate image plane. This intermediate image plane is viewed with a stereo microscope. The illumination of the fundus of the eye is carried out by a slit illumination which is coupled into the eye via the ophthalmoscoping lens. Critical disadvantages of such a fundus observation by means of a slit lamp, on the one hand, is a restriction toward a slit-shaped field of view on the retina as well as reflections of the illumination on the ophthalmoscoping lens and on a cornea of the patient which overlay the image of the fundus as interfering artefacts.
Furthermore, for the documentation of an examination or a treatment, respectively, it is necessary to store images of the fundus of the eye. To this end, for example fundus cameras and indirect ophthalmoscopes, respectively are used. In doing so, a converging lens system of the ophthalmoscope creates an intermediate image of the fundus of the eye which is imaged in an imaging plane by an imaging optics provided downstream, and in this imaging plane mostly an electronic photosensitive sensor is arranged.
With the observation of the fundus of the eye with an indirect ophthalmoscope the following parameters and boundary conditions are relevant. A field angle which is characterized by a maximum angle that two light beams starting from the eye and being imageable in the imaging plane by means of the ophthalmoscope may enclose between them, which should be as large as possible to allow a visual field of the fundus of the eye as large as possible, a size of the light sensitive sensor, and a construction size of the imaging optics of the ophthalmoscope which should be as compact as possible to keep the distance between the physician and the patient to be treated as small as possible (smaller than an arm length). In addition, relevant parameters are an imaging quality as high as possible, a diameter of the pupil of the examined eye which is smaller than 4 mm as a rule upon a preferably non-mydriatic examination, a working distance between the ophthalmoscope and the eye defined by a distance between the cornea of the eye and a surface of an entrance lens of the ophthalmoscope facing the eye which should be as large as possible so that other optical elements can be placed between the ophthalmoscope and the eye without any problems, and furthermore a larger distance between the ophthalmoscope and the nose and forehead of a patient is achieved to obtain a greater freedom of movement for the ophthalmoscope. The mentioned requirements are closely interconnected so that changes of one of these parameters have effects on the other parameters.
For example, selecting a cost-effective sensor having a diagonal of about 0.5 inch or smaller, then an intermediate image of the fundus of the eye must be usually imaged highly reduced in size onto this sensor. This either necessitates a great construction length of the imaging optics by which the distance between the physician and the patient is enlarged or the use of optical elements having short focal distances which however are in conflict with achieving a high imaging quality. To achieve a working distance as large as possible one may select an objective of the ophthalmoscope having a large focal distance. However, since the size of the intermediate image also scales with the focal distance of the objective the larger intermediate image is no longer completely detectable by the sensor with unchanged optics provided downstream. Thus, either a larger sensor has to be selected which highly enhances costs of the ophthalmoscope, or the optics provided downstream in turn as described must be able to achieve a severe reduction in size of the intermediate image which either requires a longer form of construction or the use of heavily refracting optical elements including the disadvantages mentioned above.
In addition to the creation of an intermediate image it is the object of the objective of an ophthalmoscope to allow a Maxwellian illumination upon which an apparatus pupil which usually is given by an aperture of the imaging optics is imaged into the pupil of the eye to be examined Maxwellian illumination, also referred to as pupil imaging, allows the spatial separation of an observation beam path and an illumination beam path on the cornea of the eye in the vicinity of the pupil of the eye. By this spatial separation of the mentioned beam paths it is guaranteed that reflections on the cornea of the eye and on surfaces of the objective in the fundus image do not superimpose the reflection from the fundus of the eye, i.e. the observation beam which carries the decisive image information from the eye. Furthermore, the pupil imaging allows a particularly large field angle since a particularly large region of the eye may be imaged on the sensor of the ophthalmoscope. However, pupil imaging requires an accurately adjusted working distance. If the ophthalmoscope is positioned too far from the eye the aperture is imaged in front of the eye outside of the pupil. By this, the field angle is reduced, and moreover amounts of stray light will not be removed from the observation beam path as described above.